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[TowerTalk] vertical antennas

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Subject: [TowerTalk] vertical antennas
From: (Tom Rauch)
Date: Wed, 24 Oct 2001 20:31:35 -0400
> Second, a long expanation for possible a silly question.
> I have read about underground antennas, about ground antennas that
> shorten the length of the physical wire, about radials needed when
> in/on the ground and about 10ft above the ground.  Now, if we have a

Unfortunately you will read all sorts of unreliable things when the 
data is not peer reviewed. While most of the information is well 
intentioned, the fact is most of the data is never based on accurate 
or proper measurements or data.

That's a shame, because it means a lot of effort is wasted.

> portion (still incased) up to be above the gound 10ft and bend the top
> portion to a horizontal, to resemble an "L" inverted.  This would give
> most of the current portion of the antenna above the ground, both
> sides of the dipole. This may sound silly but I donot have a program
> to simulate it, nor all the math ability, however, it still seems to
> be plausible.  I would like to hear proposals both for and against 

Even the most sophisticated modeling program has problems with 
earth, because after all it is just a model. Earth is treated as a 
homogeneous media, even though it is far from that.

The last thing in the world we want to do is concentrate fields 
(induction fields.... electric... magnetic, and radiation EM fields) in 
a narrow cross section or area of earth.

The reason a large number of radials works well close to earth is 
because they spread the fields out over a large area of earth, and 
also "shield" the lossy earth below the radials from the EM field.

There is no free lunch in this. If you have a single wire near or 
worse yet deep in earth... fields in the lossy earth are very 
concentrated, so loss is going to be *very* high.

The same is true if you fold the flat top over near earth, if there is 
not a large ground screen shielding the flat top from earth.

This is why sparse elevated radial systems are lossy, just like very 
low dipoles are over bare earth.


73, Tom W8JI 

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